1. Field of the Invention
This invention is related to the field of sensor calibration and more particularly, to a differential sensor pre-amplifier with a balanced, bi-directional calibration circuit.
2. Description of the Prior Art
Differential amplifiers are extremely useful in low-noise telemetry work because common-mode noise picked up on the input lines is canceled out by common-mode rejection. Underwater hydrophone amplifiers, however, are typically single-ended at the input because of the difficulty of injecting balanced calibration signals, while single-ended amplifiers provide only one equivalent noise source at the amplifier input.
In a typical prior art single-ended hydrophone amplifier circuit, a broadband calibration signal is applied through a series arrangement of a hydrophone element and a calibration attenuation network to only one input of an operational amplifier, the other input of the operational network being connected to signal ground. The attenuation network prevents saturation of the single-ended operational amplifier, and the injection of the calibration signal in series with the hydrophone element allows the user to verify that the hydrophone element is intact and that the rest of the electronics in the signal processing chain is working properly.
The calibration signal is usually a high-bandwidth waveform containing many tones at discrete frequencies. Injecting such a signal at the hydrophone input allows the user to verify that the hydrophone capacitive element is intact and that the rest of the electronics in the signal chain is working properly. Knowing the levels of each input calibration tone allows the user to perform a system calibration. Then, knowing the hydrophone sensitivity will enable the operator to record a precise amplitude measurement of in-water signals. The calibration signal is also a valuable way of quickly verifying system operation.
For example, as shown in FIG. 1, generally designated at 10 is a schematic circuit diagram of a typical, prior art single-ended hydrophone preamplifier with a single-ended calibration circuit. Circuit 10 includes a unidirectional hydrophone calibration circuit shown generally in dashed box 12 and a single-ended preamplifier shown generally in dashed box 14. The calibration circuit 12 and preamplifier 14, as discussed more fully below, are inherently unbalanced and do not allow calibration of the common-mode rejection feature of the hydrophone.
In the prior-art calibration circuitry, a hydrophone 16 includes positive and negative hydrophone input paths 1 and 2 respectively. Positive hydrophone input path 1 is connected to the non-inverting input of operational amplifier (op amp) 18 and negative input path 2 is connected via resistor 11 to signal ground 20. The hydrophone 16 includes a shield 3, which in turn is connected to signal ground 20.
The prior art calibration circuitry includes an input 4 and return output 5. Calibration output 5 and the calibration return signal applied thereto are connected to signal ground 20 while the calibration input 4 is applied through a resistor 13 to the hydrophone negative input 2. One end of resistor 15 is connected to the hydrophone positive input 1 and the non-inverting input 17 of op amp 18. The other end is connected to signal ground 20. Use of the resistor 15, typically called the hydrophone termination resistor, is well-known to those skilled in the art.
The preamplifier 14 provides amplification of hydrophone input signals and roll off of unwanted low-frequency noise. Resistor 19 and capacitors 21 and 23 are connected between the inverting input 26 of the op amp 18 and signal ground 20. As is well known in the art, op amp 18 can be provided with gain shaping, surge protection and other circuitry, as well as with downstream amplifiers, filtering and other stages of amplification. Additional stages of amplification are not illustrated since they have no bearing on the present invention.
In use for calibration, a broadband noise signal is applied to calibration input 4 and output 5 through the resistor divider network 11 and 13, unidirectionally through the hydrophone 16, and into the non-inverting input 17 of the op amp 18. The amplified output signal 25 of the preamplifier stage 14 is used to verify the electrical continuity of the hydrophone 16. However, the single-ended calibration circuit 12 unbalances the amplifier, and moreover, it enables the user to check the continuity of the hydrophone 16 in one electrical direction but not bi-directionally.
Because the calibration signal and the attenuation network are connected to only one input of the op amp, the calibration circuit is inherently unbalanced and could destroy any common-mode noise rejection should the amplifier be operated in its differential mode. The calibration signal can also be injected directly at the single-ended output of the amplifier, e.g. by using an analog switch; however, the user has no way of directly verifying the integrity of the hydrophone capacitive element at the input.
Operational amplifiers operating in the differential mode are extremely useful in low-noise telemetry work because common-mode noise picked up on the input lines is cancelled out by common-mode rejection. A differential-input hydrophone preamplifier with self-calibration is disclosed by Spychalski in U.S. Pat. No. 4,689,578. It includes a calibration circuit for feeding a calibration signal into one amplifier of a pair of amplifier stages that form a balanced differential input charge preamplifier at a circuit point downstream of the hydrophone element.
The gain of the balanced differential-input charge preamplifier is dependent on the capacitance of the hydrophone. The calibration circuit enables the user to calibrate the combined hydrophone/preamplifier system in situ for variation in hydrophone capacitance caused by the changing operational depth when deployed in a marine environment. However, since the calibration signal is injected downstream of the hydrophone element, there is no direct checking of the continuity of the hydrophone element itself apart from the combined hydrophone/preamplifier system. Since Spychalski injects the calibration signal downstream of the hydrophone, it does not teach providing a balanced calibration signal at the front end or input of the differential amplifier to measure the common-mode rejection capabilities of the amplifier.